Estudo de megaporfiroblastos de granada da Nappe de Passos
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2018-10-30
Autores
Hartung, Rafael Francisco [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Na Nappe de Passos, unidade tectônica pertencente à Faixa Brasília Meridional, em seu segmento intermediário (Unidades E e G), ocorrem megaporfiroblastos de granada (diâmetro ≥ 3 cm, alcançando até 8 cm), hospedados em cianita-mica xistos, que contrastam com os de tamanho usual (0,2 a 1,5 cm) comumente encontrados nos xistos granadíferos desta nappe. Este trabalho, como uma primeira iniciativa de estudo dessa peculiar feição metamórfica, investiga a ocorrência desses megaporfiroblastos através de levantamento de campo, análise microtectônica e determinação de perfis químicos. Parte do trabalho foi voltada para buscar ocorrências de megaporfiroblastos na porção NW da Nappe de Passos, a norte da cidade de Cássia (MG). Com o levantamento de campo, que incluiu trinta perfis seccionando as Unidades E, F G, H e I, não foi encontrada nenhuma ocorrência de camadas ricas em megaporfiroblastos neste setor da nappe. Ao longo desse levantamento, foi possível realizar uma melhor caracterização geométrica do fechamento NW da Braquissinformal de Passos e demonstrar o caráter braquiantiformal da Antiforma de Itaú de Minas. Para o estudo dos megaporfiroblastos, selecionou-se uma ocorrência no segmento SE da Nappe de Passos, no topo da Unidade E. Eles se encontram hospedados em camadas decimétricas de biotita-cianita-granada-quartzo-muscovita xistos, que se alternam com camadas de composições semelhantes, porém contendo porfiroblastos de tamanho usual. Os megaporfiroblastos têm forma elipsoidal, com eixo maior paralelo à lineação mineral associada à foliação principal (S2). A análise de microestruturas em quarenta e seis megaporfiroblastos mostra trilhas de inclusões definindo foliação interna (Si) retilínea a suavemente ondulada no centro, passando a crenulada nas bordas. A Si (=S1//S0) não apresenta continuidade com foliação principal (Se=S2), que se amolda ao redor dos megaporfiroblastos, evidenciando crescimento sincinemático, cedo-D2. A ausência de continuidade entre Si e Se e a posição variada dos planos axiais das crenulações inclusas, indicam prosseguimento da deformação após cessar o crescimento dos porfiroblastos, cujas sombras de deformação assimétricas revelam regime de deformação não-coaxial associado a D2, coerente com o sentido de transporte da nappe (topo para SE). Os megaporfiroblastos são ricos em almandina (73 a 82%) e pobres em espessartita (0,1 a 2,5%) e guardam zonação de crescimento com incremento de almandina e piropo para a borda, acompanhado da redução de espessartita e grossulária, interpretada como blastese sob aumento de temperatura. Os porfiroblastos de tamanho comum de mica xistos, também da unidade E mostram essencialmente o mesmo padrão de inclusões e de zonação química, indicando que compartilham a mesma evolução tectonometamórfica. Os dados levantados indicam que a formação dos megaporfiroblastos deve estar relacionada à composição altamente aluminosa do protólito, associada a escassez de pontos de nucleação.
The Passos Nappe is an important tectonic unit of the Southern Brasília Belt. In the intermediate segment of the Passos Nappe, represented by E and G Units, occurs megaporphyroblasts of garnet (diameter 3 to 8 cm). The megaporphyroblasts are hosted in kyanite-mica schists and contrast with the usual size (from 0.2 to 1.5 cm) commonly found in the granadiferous schists of this nappe. This work provides the first study of this peculiar metamorphic feature and aims to investigate the occurrence of these megaporphyroblasts based on field work, microtectonic analysis and determination of chemical profiles. Part of the work focused on identifying the occurrence of megaporphyroblasts to the north of the Cássia city (Minas Gerais State), within the NW sector of the Passos Nappe. Thirty transects were produced along the E, F G, H and I Units, however, no occurrence of megaporphyroblasts rich layers was found in this sector of the nappe. The improvement on the geometric characterization of the northwest closure of the Passos Brachisyncline and the demonstration of the brachianticline aspect of Itaú de Minas Antiform were possible with the transect work. For the study of megaporphyroblasts, it was selected an occurrence located in the SE sector of the Passos Nappe, situated at the top of the E Unit. The study carried out in this sector shows that megaporphyroblasts are hosted in decimetric layers of biotite-kyanite-garnet-quartz-muscovite schists, which alternate with layers of similar compositions, but containing porphyroblasts of usual size. The megaporphyroblasts have ellipsoidal shape, with the major axis parallel to the mineral lineation, associated with the main foliation (S2). The analysis of microstructures in forty-six megaporphyroblasts shows inclusion trails that define an internal foliation (Si) that is straight to gently undulating in the center of the garnet, passing to crenulated at the rims. The Si (=S1//S0) does not exhibit continuity with the main foliation (Se=S2), which curves around the megaporphyroblasts, suggesting synkinematic growth early D2. The absence of continuity between Si and Se and the varied position of the axial planes of the included crenulations, indicate continuation of the deformation after the growth of the porphyroblast, whose asymmetric deformation shadows reveal a non-coaxial D2 deformation regime that is consistent with the nappe transport (top for SE). The megaporphyroblasts are almandine-rich (73 to 82%) and spessartite-poor (0.1 to 2.5%) and they show growth zonation with increase of almandine and pyropo to the rims, accompanied by reduction of spessartite and grossular. This is interpreted as an increasing temperature blastesis. The common size garnet porphyroblasts of the mica schists, also of the E Unit, show essentially the same inclusions pattern and chemical zonation like the megaporphyroblasts, indicating that they share the same tectonometamorphic evolution. The collected data indicate that the formation of megaporphyroblasts must be related to the high-Al protolith composition, associated with scarce nucleation points.
The Passos Nappe is an important tectonic unit of the Southern Brasília Belt. In the intermediate segment of the Passos Nappe, represented by E and G Units, occurs megaporphyroblasts of garnet (diameter 3 to 8 cm). The megaporphyroblasts are hosted in kyanite-mica schists and contrast with the usual size (from 0.2 to 1.5 cm) commonly found in the granadiferous schists of this nappe. This work provides the first study of this peculiar metamorphic feature and aims to investigate the occurrence of these megaporphyroblasts based on field work, microtectonic analysis and determination of chemical profiles. Part of the work focused on identifying the occurrence of megaporphyroblasts to the north of the Cássia city (Minas Gerais State), within the NW sector of the Passos Nappe. Thirty transects were produced along the E, F G, H and I Units, however, no occurrence of megaporphyroblasts rich layers was found in this sector of the nappe. The improvement on the geometric characterization of the northwest closure of the Passos Brachisyncline and the demonstration of the brachianticline aspect of Itaú de Minas Antiform were possible with the transect work. For the study of megaporphyroblasts, it was selected an occurrence located in the SE sector of the Passos Nappe, situated at the top of the E Unit. The study carried out in this sector shows that megaporphyroblasts are hosted in decimetric layers of biotite-kyanite-garnet-quartz-muscovite schists, which alternate with layers of similar compositions, but containing porphyroblasts of usual size. The megaporphyroblasts have ellipsoidal shape, with the major axis parallel to the mineral lineation, associated with the main foliation (S2). The analysis of microstructures in forty-six megaporphyroblasts shows inclusion trails that define an internal foliation (Si) that is straight to gently undulating in the center of the garnet, passing to crenulated at the rims. The Si (=S1//S0) does not exhibit continuity with the main foliation (Se=S2), which curves around the megaporphyroblasts, suggesting synkinematic growth early D2. The absence of continuity between Si and Se and the varied position of the axial planes of the included crenulations, indicate continuation of the deformation after the growth of the porphyroblast, whose asymmetric deformation shadows reveal a non-coaxial D2 deformation regime that is consistent with the nappe transport (top for SE). The megaporphyroblasts are almandine-rich (73 to 82%) and spessartite-poor (0.1 to 2.5%) and they show growth zonation with increase of almandine and pyropo to the rims, accompanied by reduction of spessartite and grossular. This is interpreted as an increasing temperature blastesis. The common size garnet porphyroblasts of the mica schists, also of the E Unit, show essentially the same inclusions pattern and chemical zonation like the megaporphyroblasts, indicating that they share the same tectonometamorphic evolution. The collected data indicate that the formation of megaporphyroblasts must be related to the high-Al protolith composition, associated with scarce nucleation points.
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Palavras-chave
Faixa Brasília, Nappe de Passos, Megaporfiroblastos de granada, Microtectônica, Granada zonada, Brasília Belt, Passos Nappe, Garnet megaporphyroblasts, Microtectonics, Garnet zoned